Optical fiber networks are increasingly important for the distribution of voice, video, and data signals. Such systems generally involve a number of feeder fibers that emanate from a headend office, and terminate at respective remote terminals. In a Fiber-To-The-Home or a Fiber-To-The-Curb system, optical signals are transmitted from each of these remote terminals to a number of optical network units (ONUs) over distribution fiber. Signals are transmitted optically or electrically to each ONU.
Network architectures have been proposed for transmitting signals between the headend office and the ONUs. One popular architecture uses a passive optical branching device to exchange signals between the feeder and distribution fibers, and this is particularly desirable because power is not required. Among the branching devices that are available, one in particular has emerged as potentially the most useful of all. This device combines signal broadcasting with signal distribution, and is symbolically and functionally shown in FIG. 1. Because each function is generally handled by a separate passive optical network (PON), this device has become known to as "2-PONs-In-1," or 2P1 for short. Simply stated, the 2P1 device overlays a power splitter (PS) PON and a wavelength-division multiplexer (WDM) PON on the same optical integrated circuit. The WDM PON can be used to send private signals to each subscriber, while the PS PON can be used simultaneously to broadcast signals. Alternatively, in a single-fiber bi-directional network, the WDM PON can be used for downstream, and the PS PON for upstream. In addition, the network can be initially deployed as a public broadcast network using the PS PON, and upgraded at a later time to a private distribution network using the WDM PON without changing outside plant. Furthermore, in a power splitting network using the 2P1 device, the WDM overlay can be used for "lit"-fiber testing (i.e., testing a particular fiber at one wavelength without disturbing active signal transmission at a different wavelength).
Optical multiplexing and demultiplexing have been accomplished in the past via a pair of star couplers that are interconnected by an optical grating (i.e., a number of waveguides--each differing in length with respect to its nearest neighbor by a predetermined fixed amount). Such structures are frequently referred to as dense WDMs, which separate optical transmission into many narrow wavelength bands (channels)--as contrasted with coarse WDMs, which separate optical transmission into two relatively wide channels. Examples of dense WDMs that use interconnected star couplers are shown in U.S. Pat. Nos. 5,002,350 and 5,136,671. In one direction of optical transmission, the dense WDM can be used as a multiplexer wherein a plurality of separate and distinct wavelengths (.lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.n) are launched into different input ports of one star coupler and emerge on a single output port of the other star coupler. In the other direction of optical transmission, the dense WDM can be used as a demultiplexer wherein a plurality of different wavelengths are launched into a single port of one star coupler and emerge on multiple ports of the other star coupler according to their particular wavelengths. Accordingly, dense WDMs are often referred to wavelength routing devices, and the wavelength region (band) that is routed to/from a particular port is referred to as a channel.
U.S. Pat. No. 5,412,744 discloses a dense WDM having a power splitter connected in series with the input ports of the dense WDM. However, this structure is not suitable for use as a 2P1 device because the power splitter disclosed does not broadcast optical power to all of the output channels. Instead, it behaves like another WDM channel, albeit with a wider and flatter passband.
U.S. Pat. No. 5,321,541 discloses a 2P1 device that functions transparently as a dense WDM at 1550 nanometers (where 1 nanometer=1 nm=a billionth of a meter) and as a power splitter at 1310 nm. These two wavelength regions are first separated by a coarse WDM. Thereafter, optical signals in the 1550 nm region are routed around the power splitter, which broadcasts optical signals in the 1310 nm region. These parallel signals are then recombined by coarse WDMs at each output port of the 2P1 device. This particular design is implemented on a single chip using conventional silica waveguide technology; but it requires numerous waveguide crossings and two stages of coarse WDMs that add loss (about 1.5 dB) and crosstalk. This particular design is shown herein as FIG. 3.
U.S. Pat. No. 5,440,416 discloses another 2P1 device based on a dense WDM having reflective waveguides. In one embodiment of this design, the mirror symmetry of a conventional dense WDM is used. A reflective coating is applied at the mirror plane, and only that portion of the WDM on one side of the mirror plane is retained. In another embodiment, the reflective coating is replaced with Bragg reflectors, which are gratings that can be delineated by photolithography. However, the additional processing required is not cost effective because it not only takes additional time, but it also decreases yield if not executed precisely.
The '416 patent discloses yet another embodiment of a 2P1 device wherein the waveguide carrying the broadcast optical signal (for delivery to all output ports) is connected to the second stage of a dense WDM. However, because an array of waveguides (gratings) must also be connected to the second stage, the broadcast waveguide cannot be properly positioned. Ideally, the broadcast waveguide is positioned at the center of the gratings; but this would either require waveguide crossings at non-optimum angles, or a multi-layer arrangement with increased processing cost. Unfortunately, by positioning the waveguide that carries the optical signals to be broadcast at the end of the grating array, as shown by FIG. 4 herein, optical signal power output is decreased. In other words, coupling efficiency is reduced.
What is desired, and what does not appear to be disclosed in the prior art, is an optical multiplexer/demultiplexer for use in a 2P1 device, but having improved coupling efficiency over the prior art. Moreover, the desired optical multiplexer/demultiplexer should avoid waveguide crossings and multi-layer construction.